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13mm T6 vs. Pentax 14mm XW

Posted by Brian Carter   08/07/2005 12:00AM

Well, here are the stats:

Discover 10" F/5,6 dob
14mm pentax = 101x with 41.5' FOV
13mm T6 = 109x with 45' FOV

There is about an 8% difference in magnification, which is extremely little, I doubt it will be much of a big deal, with 10" aperture, I think it will be hard to see a difference without there being A/B comparisons.

Finally... The 13mm nagler was my first Nagler, well, not really, the 7mm T1 was, but I didn't like it, but I had the 13 for a long time and enjoyed it, I just found the 14mm to be nicer, mainly for the eye relief, though. But I just want to look at sharpness, contrast, aberrations, distortion, etc. Not really interested here in ease of use because that is subjective and I've already made up my mind (obviously) for that.

I had about an hour tonight and got a bunch done. Only had the moon and jupiter to look through the trees, but at least it was a clear look through no trees or limbs over the house. Scope had been cooling all night; I could have looked at other things but I had a horrible date tonight and all I could think of the whole time was how I'd rather be looking at saturn... oh well.

13mm T6 vs. 14mm XW - The Nagler wins, hands down. It is very flat, all the way to the edge, focus in the center was only slightly less sharp at the edge. With Jupiter et al in the center, Ganymede was nearer the edge and it was a nice comparison with the other moons. It lost very little sharpness at the edge. Lateral color was kind of a problem, the outer-most side of Jupiter turning a little green and the inside turning purplish/maroon on the inside around 50% out from center. The biggest problem was some flattening and squishing of jupiter which really became noticeable at 80% out. This wasn't really bad, not as bad as a panoptic, and i'd like to see how much it is panning around the milkyway - but since I don't pan much at 100x, i don't think it would ever matter.

14mm Pentax XW: Had a lot better color correction. Outer limb starts turning green and objectionable at about 75% out. No little squishing distortion that I could tell. If any, it is surely not near as bad as the nagler. At about 50% out field curvature starts getting bad. Again, with Jupiter et al. were in the center in focus, poor Ganymede was pretty out of focus. But 80% out, I could start to see the darkening of the secondary. There is some edge astigmatism, but not a lot. Defocusing at the edge shows a little fan shaped coma, which is interesting because it is absent in the nagler which shows a larger true field.

On the moon, the pentax wins a little on contrast. The blacks were blacker. Two little 'dots' of sunlight white peeks in a shadowed crater was striking in the 14mm but had to look for it with the 13mm. My other Pentaxes showed the two peaks no problem. Also on Jupiter, those little low contrast blue festoons around the equator? Caught them easier in the Pentax than the nagler, and easier in the other pentaxes.

There is a little less scatter in the pentax.

Nagler is sharper on axis very noticeably, which is interesting because it also gives slightly more magnification.

So conclusions for those who skipped all that:

The 13mm T6 nagler is sharper than the 14mm Pentax XW, shows little-to-no field curvature while the pentax shows a good bit, but has a little more color off axis. On axis the nagler wins too. But the pentax shows better contrast, easily noticed on the moon.

So, since I am an open/globular cluster kind of guy, I may not be keeping the 14mm XW for very much longer, I'm going to wait to check it out on some galaxies and nebulae... but for OC/GCs, I need a flat field, which the 14mm just doesn't give.

HOWEVER: this negative trait is not shared in the 10mm, 7mm XWs or the 10.5mm XL, which have very flat fields at F/5,6.

Click here for more about the Nagler 13mm Type 6. -Ed.

Click here for more about the 14mm Pentax. -Ed.